English translation of the book indicated above Nihon Tō Kōza, Volume VI, Part 1 by Harry Afu Watson, AFU Research Enterprises, Inc., 1993. Tsuba in question illustrated on page 14 and described as follows: " Tsuba mumei Ōnin. Tetsu ji maru gata ko-sukashi tsuchime shitate shinchū suemon ten zogan maru mimi. Brass suemon". My question remains: why such a text is called 'translation' while it looks more like transliteration of romanization? 
Gary D. Murtha's tsuba on page 61.
Tsuba of oval form decorated with clematis six-petal flowers, tendrils, and leaves in cast brass with details carved in kebori, inlaid on iron plate carved in low relief (kebori and sukidashi-bori). Hitsu-ana plugged with shakudō. Copper sekigane.
Heianjō (or Ōnin) school. Unsigned. Mid Muromachi period (1454-1513). Dimensions: 87.2 x 84.3 x 4.3 mm. Tsuba is illustrated and described in Gary D. Murtha's "Onin-Heianjo-Yoshiro" book on pages 38-39. Mid-Muromachi is the age attribution by Gary. “A picture book of Japanese sword guards. Victoria & Albert Museum“, published in 1927 presents us with a somewhat similar tsuba: "Floral ornament. Iron, with brass incrustation". V&A attributes the tsuba to Ōnin style, 16th century.
Signed: Nobuie [信家] / 62
Probably the work of Shodai Nobuie (c. 1580).
Tokubetsu hozon certificate № 2002993 of the N.B.T.H.K., dated January 15, 2016. NOBUIE TSUBA by Steve Waszak The iron tsuba made by the two early Nobuie masters are regarded as the greatest sword guards ever made across hundreds of years of Japanese history. Only a small handful of other smiths' names are even mentioned in the same breath as that of Nobuie. Despite the well-deserved fame of the Nobuie name, virtually nothing is known with certainty about the lives of the two men who made the pieces carrying this name. They are thought to have been men of Owari Province, with the Nidai Nobuie also spending time in Aki Province at the end of the Momoyama Period. Two Nobuie tsubako are recognized. The man whom most consider to have been the Shodai signed his sword guards with finer and more elegantly inscribed characters than the smith seen by most as the Nidai. The term used to describe the mei of the Shodai is "hanare-mei" or "ga-mei," while that used to characterize the signature of the Nidai is "futoji-mei" or "chikara-mei." These terms refer to the fineness and grace of the Shodai's signature and the relatively more powerfully inscribed characters of the Nidai's. The Shodai is thought to have lived during the Eiroku and Tensho eras in the latter part of the 16th century, while the Nidai's years are considered to have been from Tensho into the Genna era. This locates both smiths well within the Golden Age of tsuba artists -- the Momoyama Period. Nobuie tsuba are esteemed and celebrated for the extraordinary beauty of their iron. The combination of the forging of the metal, the surface treatment by tsuchime and yakite married to powerfully expressive carving, the masterful manipulation of form, mass and shape, and the colour and patina of the iron makes Nobuie sword guards not only unique in the world of tsuba, but the greatest of the great. The sword guard here is a Shodai-made masterwork, done in mokko-gata form, a shape the early Nobuie smiths mastered to a degree unmatched by any others. The expanding of the mass of the tsuba from the seppa-dai to the mimi, increasing by 50% from the centre of the guard to the rim, creates a sense of exploding energy, which is then contained by the uchikaeshi-mimi, yielding a lightning-in-a-bottle effect of captured energy. The hammering the master has employed to finish the surface is subtle and sensitive, achieving a resonant profundity, and the deep blue-black colour -- augmented by a lustrous patina -- leaves the tsuba to positively glow in one's hand. In this piece, Nobuie has used a motif of several kamon, or family crests, each carved only lightly on the surface in a loose ring around the nakago-ana. Due to the shallow depth of this carving, together with the tsuchime finish of the plate, the effect is to leave the kamon with a sort of weathered appearance, recalling the prime aesthetic values of sabi and wabi, which had great circulation in the Tea Culture so ascendant in the Momoyama years. However, the effects of sabi and wabi expressed in the treatment described above are amplified and deepened by the color and patina of the iron, thereby adding yet another aesthetic value -- yuugen -- which is linked with the abiding mystery of the universe and one more — mono no aware — which alludes to the pathos of life's experiences and transitory nature. In short, this Nobuie tsuba joins poetry with power and therein exemplifies the unrivalled brilliance of Nobuie workmanship.
Iron tsuba of round form pierced (sukashi) in a chessboard fashion and decorated with linear (sen-zōgan) and cast (suemon-zōgan) brass inlay, including symbols of the swastika, flower-lozenge, maple leaf, pine needle, etc. on both sides; rim and openings outlined with brass inlay. Nakagō-ana plugged with copper fittings (sekigane).
Momoyama period. End of the 16th - beginning of the 17th century. Dimensions: Diameter: 75.5; Thickness: 4.5 mm.
Tsuba of oval form decorated with vines, tendrils, and leaves on trellis in brass inlay with details carved in kebori, and pierced with six family crests (mon) with two, three and four pointing stars in openwork, each outlined with brass wire and carved in kebori. Original hitsu-ana outlined with brass wire was probably enlarged later. Copper sekigane.
Momoyama to early Edo period (end of the 16th - beginning of the 17th century). Dimensions: 68.3 x 64.5 x 3.4 mm.
TSU-0282: Ko-kinko yamagane tsuba with waves and rabbit motif.
Other similar specimens can be found at:
Henri L. Joly and Kumasaku Tomita, Japanese art and handicraft, "Swords and sword fittings" section, sub-section “Inlays of Ōnin, Kyoto, Fushimi-Yoshiro, and Kaga Province”, Plate CX, #128: Iron, chrysanthemoid, thin guard with alternate petals covered with brass spots. Ōnin style. 16th century.
Japanese art and handicraft, Plate CX, #128.
Compton Collection, Part I, #7.
Butterfield & Butterfield, 1979. Sale # 3063, lot № 66.
Size: 80.4 x 75.8 x 5.2 mm
NBTHK Certificate №454567, allegedly saying that it is Akasaka School, Muromachi period. A look-a-like tsuba in Robert. E. Haynes Catalog #7, 1983 on page 57 under №48 is described as follows: "A masterpiece second period Owari sukashi tsuba. The plate is of beautiful color and quality almost like velvet. The design is very hard to discern, it might be the horse trappings, or even a moon. The style and type of Owari tsuba shows the great tradition of the Momoyama period and why it was the renaissance in time, as well as the arts produced, through the long history of all Japanese art. Ca. 1580. Ht. 7.7 cm, Th. (center) 5.5 mm, (edge) 5.25 to 5.75 mm."
Robert. E. Haynes Catalog #7, 1983, p. 57, №48.
Small iron tsuba for a dagger (tantō), of quatrefoil form (mokkō-gata), with raised rim (mimi), decorated with flat brass inlay (hira-zōgan) to form an abstract design alluding to the mushroom of immortality (reishi). Opening (hitsu-ana) to the left of nakaga-ana probably cut later and fitted with shakudo sekigane. Maker's signature on seppa-dai: Koike Naomasa (小池 直正).
Momoyama period: End of the 16th - beginning of the 17th century. Dimensions: Height 53.7 mm; Width: 45.5 mm; Thickness at centre: 3.5 mm; at rim: 4.9 mm. Other examples of signed Koike Naomasa work in this collection: TSU-0346. Reference: The closest example in literature is in Compton Collection (II): №11 with the description: “A Koike School tsuba, Edo period (circa 1625), signed Koike Yoshiro. Sheet-brass flush inlay of cloud forms and wire inlay creating the same shape. Koike Yoshiro Naomasa worked from the Keicho to the Genna periods (1596-1623). He arrived in Kyoto from Kaga.” [Japanese Swords and Sword Fittings from the Collection of Dr. Walter Ames Compton (Part II) / Sebastian Izzard, Yoshinori Munemura. — Christie's, New York, October 22, 1992]. See: Yoshirō tsuba.
Iron tsuba of square with cut-off edges form (sumi-iri-kakugata) with lattice design in openwork (sukashi) and solid center. Hitsu-ana plugged with lead.
Unsigned. Late Muromachi period, ca. 16th century.
Size: 81.3 x 80.0 x 3.6 mm References: 1) Tsuba Kanshoki. Kazutaro Torogoye, 1975, p. 95, lower image. It's also called Kyō shōami. 2) KTK-11: Koshi motif, Late Muromachi (16th c.)
Jeanne Allen. Designer's guide to Samurai Patterns. Chronicle Books, San Francisco, 1990. Page 114, №130.
Iron tsuba of round form decorated with a design of bracken scrolls and paulownia leaves and blossoms (kiri-mon) in openwork (sukashi). Details carved in kebori. Squared rim with iron bones (tekkotsu). Hitsu-ana plugged with shakudō.
Size: 83.6 x 82.9 x 5.4 (center), 5.1 (rim) mm.
Unsigned.
Muromachi period, ca. 16th century.
Tsukioka Yoshitoshi [月岡 芳年] (Japan, 1839 – 1892): Tokugawa Yoshimune [徳川 吉宗] (1684 – 1751) playing kemari [蹴鞠]
Iron tsuba of square with cut-off edges form (sumi-iri-kakugata) with lattice design in openwork (sukashi) and pierced center.
Unsigned. Late Muromachi period, ca. 16th century.
Size: 73.2 x 72.4 x 3.6 mm References: 1) Tsuba Kanshoki. Kazutaro Torogoye, 1975, p. 95, lower image. It's also called Kyō shōami. 2) KTK-11: Koshi motif, Late Muromachi (16th c.)
The Henry D. Rosin Collection №9.
Lundgren Collection №7.
Iron tsuba in a form of an eight-petalled blossom (lotus) form, petals separated by linear low-relief carving, both hitsu-ana filled with gold plugs, the surface decorated with tsuchime-ji, rich grey-brownish patina, niku from 4 mm in the centre to 6 mm at the rim. Strong (futoji-mei) Nobuie [信家] signature to the left of nakago-ana. Attributed to the 2nd generation of Nobuie masters (Nidai Nobuie).
Size: outer diameter 84 mm, thickness at centre: 4 mm, at rim: 6 mm. Wight: 167 g.Signed: Nobuie [信家]
Probably the work of Nidai Nobuie (c. 1600).
The gold plugs are likely a later work.